Each time the clutch is engaged, the friction pads become progressively gripped between a pressure plate and a reaction plate of the clutch. The heat generated by the friction in the clutch leads inevitably to the pressure plate gradually assuming a conical form. The same is true of the reaction plate, even though its conicity is not so pronounced as for the pressure plate. As a result, the interface zone in which each friction pad is subjected to pressure from the adjacent reaction or pressure plate, as the case may be, progressively moves towards the axis of the assembly. Consequently the friction pads are subjected to uneven wear and a reduction in their effectiveness. This can even lead to possible breakage of the friction pads.
In some friction discs, some kind of suitable resilient means are provided between the two portions of a friction pad in order to give a certain degree of progressive engagement of the clutch. These resilient means can, at least to some extent, reduce the drawback mentioned above, by allowing some play between the two portions of the friction pad, and thus by following any tendency of the reaction and pressure plates to adopt a conical profile.
However, in order to give an effective progressive engagement of the clutch, these resilient means must be relatively stiff, whereas they should be of relatively low stiffness to enable there to be a satisfactory amount of play or deformation between the portions of each pad. To some extent these requirements are found to be incompatible with each other.